Published Tuesday, February 15, 2000  in the San Jose Mercury News


Air flows like water, with eddies of warmth, chill

Special to the Mercury News

Q. When I run in cold weather, occasionally I pass through a pocket of warm air for maybe a half-second -- I can feel it. And it is not because I am running into sunlight from shade, or wind, or a change in terrain where heat absorption of the ground surface may come into play. It also happens when I'm running in the heat and suddenly feel a pocket of cool air for
10 feet or so. What causes this? John Formale - San Jose

A.  Without a lot more information, I can make only an educated guess. Keep in mind that the atmosphere acts like a fluid, with eddies, swirls, convection, conduction and other subtle motions. I think you are moving through a pocket of air that was warmed or cooled nearby and then moved horizontally -- advection is the technical term -- to your

Q.  Why do hurricanes develop only over water?  Samuel Tully - Redwood City

A. The main ingredient that makes hurricanes powerful storms is the energy of the warm waters over which they form. The water temperature must be more than 80 degrees. The only places where that occurs are in the tropics between about 5 degrees and 25 degrees north and south latitude, and in portions of the Gulf Stream at higher latitudes. While the land can be very warm in the tropics, it does not have the moisture needed to develop a hurricane. And the terrain disrupts the low-level wind patterns necessary for a hurricane to survive.

Q.  We think of the atmosphere becoming saturated at 100 percent relative humidity, at which point condensation should occur. But in some cases, the atmosphere must become super-saturated -- such as 102 percent relative humidity -- before there is condensation. Why don't we get condensation at 100 percent?  Doug Hall - Sunnyvale

A.  Calculation of the point at which air is 100 percent saturated is based upon condensation on a flat surface. However, because water droplets are curved surfaces, the amount of water vapor in the air needs to be slightly higher for condensation to occur. And the tinier the droplet, the greater the relative humidity needed for condensation. For example, for very small cloud droplets, the relative humidity needed for condensation is approximately 100.2 percent.

Q.   Are global climate changes responsible for some of the unusual weather in Southern California in the recent past? For instance, the Los Angeles basin seems to be in the midst of a drought (excluding, of course, the rainy El Niņo years). Although record books show L.A.'s average annual rainfall to compare well to San Jose's, its rainfall has been significantly
less than San Jose's almost every year in the recent past -- again, except for El Niņo years.  Are there indications that Los Angeles is headed for a pervasive drought? S. Nolan - San Jose

A.  Global climate change is not something we can detect in 10 or even 20 years. During these relatively short periods, there is enough natural variation that it's impossible to separate what is really change from what is just a temporary noise on the climatic signal. In the past 10 years, both Los Angeles and San Jose have had five years of above-normal rainfall and five
years below average. During that period, Los Angeles averaged 15.28 inches per year, compared with its 50-year average of 14.90 inches. San Jose has averaged 15.65 inches over the past 10 years, compared to a 50-year average of 14.43 inches.

More important, as you noted, is the short-term impact of La Niņa, which typically means below-normal rainfall in Southern California.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone and fax them at (510) 657-2246 or e-mail them to